1. Field of the Invention
The present invention relates generally to optical fiber and, more particularly, to the measurement of optical power.
2. Related Art
Optical fiber cables and networks are well known in the art and generally employ optoelectric circuits. These circuits convert an electrical signal to an optical signal (e.g., an optical transmitter) or convert an optical signal to an electrical signal (e.g., an optical receiver).
For example, FIG. 1 shows a conventional fiber optics receiver 100 that includes a photodiode 106, amplifiers 108 and 110 (e.g., operational amplifiers), and an N-bit analog-to-digital (A/D) converter 112. Photodiode 106, coupled to an optical fiber 102, converts an optical signal 104 into electrical signals. Amplifiers 108 and 110 receive the electrical signals from photodiode 106 and provide a direct current voltage signal (VDC) and an alternating current serial data stream (VAC), respectively. The direct current voltage signal (VDC) is proportional to the average optical power received by photodiode 106 and is converted by A/D converter 112 into a digital output code (DOUT) of N-bits that is also proportional to the received optical power.
In a typical application, A/D converter 112 is required to provide the digital output code (DOUT) for received optical power over a 40 dBm range, such as from 1 μW to 10 mW for example. Consequently, A/D converter 112 must provide a 14-bit (i.e., N equals 14) result to resolve one part in ten thousand to support the four orders of magnitude of dynamic range. However, the accuracy required is typically only one percent, which normally implies a 7-bit result. Therefore, at the upper end of the measurement range, the seven least significant bits (LSB) of the 14-bit result are not required. At the lower end of the measurement range and assuming A/D converter 112 provides 14 bits with an accuracy of +/−0.5 LSB, it can be shown that the measurement error is approximately thirty percent. As a result, there is a need for an improved system and method for measuring optical power.